Treatment for virgin phosphorous/vanadium oxidation catalysts

ABSTRACT

A method of removing chloride from phosphorus/vanadium/oxygen mixed oxide oxidation catalysts comprising treating a chloride containing catalyst comprising phosphorus, vanadium and oxygen with a stream of gas comprising oxygen, steam and an inert gas at flow rate and temperature and for a period of time to substantially reduce the amount of chloride in the catalyst.

This is a continuation of application Ser. No. 08/024,320, filed Mar. 1,1993 now U.S. Pat. No. 5,348,927.

BACKGROUND OF THE INVENTION

The present invention relates to a treatment for virgin PVO catalystused in the partial oxidation of hydrocarbons to prepare dicarboxylicacids and anhydrides. The present invention is directed to PVO catalystprepared by procedures that employ an HCl reduction step to produce thereduced vanadium. Most particularly, the invention relates to a processfor reducing the chloride content in the PVO catalyst.

Basically, all of the methods used to prepare oxidation catalysts seekto obtain vanadium in a valence state of less than +5. One method ofachieving this is to begin with vanadium in less than the +5 valencestate. Another method and that used most widely in the art is to startwith vanadium in the +5 state and reduce the valency to less than +5.This invention relates to the latter method. Several variations on thismethod have been used to obtain these catalyst. In one method V₂ O₅ isreduced in a solution with HCl to obtain vanadyl chloride. A typicalcatalyst preparation may involve dissolving the vanadium, phosphorus,and other components in a common solvent. The reduced vanadium with avalence of less than 5 is obtained by initially using a vanadiumcompound with a valence of plus 5 such as V₂ O₅ and thereafter reducingto the lower valence with,, for example, hydrochloric acid during thecatalyst preparation to form the vanadium oxysalt, vanadyl chloride, insitu. The vanadium compound is dissolved in a reducing solvent, such ashydrochloric acid. The solvent functions not only to form a solvent forthe reaction, but also to reduce the valence of the vanadium compound toa valence of less than 5. Preferably, the vanadium compound is firstdissolved in the solvent and thereafter the phosphorus and othercomponents, if any, are added. The reaction to form the complex may beaccelerated by the application of heat. The complex formed is then,without a precipitation step, deposited as a solution onto a carrier anddried. Generally, the average valence of the vanadium will be betweenabout plus 2.5 and 4.6 at the time of deposition onto the carrier.

In another method the catalyst is prepared by precipitating the metalcompounds, either with or without a carrier, from a colloidal dispersionof the ingredients in an inert liquid. In some instances the catalystmay be deposited as molten metal compounds onto a carrier. The catalystshave also been prepared by heating and mixing anhydrous forms ofphosphorus acids with vanadium compounds and other components. In any ofthe methods of preparation, heat may be applied to accelerate theformation of the complex.

A method of obtaining vanadyl chloride was disclosed by Koppel et al,Zeit. anorg. Chem, 45, p. 346-351, 1905 by the reduction of V₂ O₅ inalcoholic HCl solution. This method has been recommended for thepreparation of the phosphorus-vanadium oxidation catalyst for example,by Kerr in U.S. Pat. No. 3,255,211 where the solvent also serves as thereducing agent. Subsequently, U.S. Pat. Nos. 4,017,521; 4,043,943;4,251,390; 4,283,307 and 4,418,003 for example, employed this methodgenerally referred to as the "anhydrous process" of reducing vanadium toprepare the basic phosphorus-vanadium catalyst. The catalysts producedby this latter method have been found to be generally superior tosimilar catalyst by the other methods. Specifically what had occurred tothis class of oxidation catalysts prior to the return to the anhydrousprocess had been the addition of a veritable cornucopia of elements tothe base vanadium-phosphorus composition, see for example U.S. Pat. No.4,105,586 where in addition to V, P and O the catalyst must contain nineother elements. The catalyst were satisfactory, but manufacturing wasdifficult because of the number of components and their varying effectson the catalyst performance.

Many references disclose oxidation catalysts which are suitable forproducing maleic anhydride by the partial oxidation of n-butane, whichcatalysts contain molybdenum as one component of a phosphorus, vanadiummixed oxide catalyst. For example U.S. Pat. No. 3,980,585 discloses acatalyst containing P, V Cu and one of Te, Zr, Ni, Ce, W, Pd, Ag, Mn,Cr, Zn, Mo, Re, Sn, La, Hf Ta, Th, Ca, U or Sn; and U.S. Pat. No.4,056,487 discloses a PVO catalyst containing Nb, Cu, Mo, Ni, Co andplus one or more of Ce, Nd, Ba, Hf, U, Ru, Re, Li or Mg. U.S. Pat. No.4,515,904 discloses a procedure for preparing PVO catalysts which mayinclude one metal of Mo, Zn, W, U, Sn, Bi, Ti, Zr, Ni, Cr or Co inatomic ratios of metal: V of 0.001 to 0.2:1.

U.S. Pat. No. 4,147,661 discloses high surface area PVO mixed oxidecatalyst additionally containing W, Sb, Ni and/or Mo at atomic ratios of0.0025 to 1:1 to vanadium.

U.S. Pat. No. 4,418,003 discloses PVO catalysts containing either Zn orMo which is deactivated by Na or Li and which may also contain Zr, Ni,Ce, Cr, Mn, Ni and Al.

Commonly assigned U.S. Pat. No. 5,070,060 discloses an oxidationcatalyst which contains molybdenum which produces a more stablecatalyst.

The use of an HCl reduction step to produce the reduced vanadium from +5vanadium is the most widely used commercial procedure for preparing thePVO catalysts. Even after calcination to prepare the catalyst, residualchloride ions remain in the virgin catalyst. As the term is used here"virgin" or "fresh" PVO catalyst refers to a catalyst that has not beenactivated for use or used in a partial oxidation process.

The chloride previously were removed during the catalyst activationperiod in the reactor, but their release from the solid catalyst in tothe reactor and the downstream equipment in the process can cause severproblems. The main problems are: equipment corrosion and coloredproducts, which in turn results in poor product quality and/or productloss and eventually producing an increased waste disposal. Thus, theproblem is to remove the chloride at the point of catalyst manufactureor at least before it is exposed to hydrocarbon feed in the reactor.However, any procedure that is employed to remove the chloride prior toactivation must not result in a detrimental change in the catalyst perse and in particular not oxidize or reduce the vanadium component of thevirgin catalyst, which preferably has a valence of around 4+ in virgincatalyst.

It is an advantage of the present invention that the chloride can beremoved from the virgin PVO catalyst without detriment to the catalyst.It is a further advantage that the valence of the vanadium remains inthe preferred range. These and other advantages and features will becomeapparent from this disclosure. It is a particular advantage that thepresent method is especially suitable for removal of high percentages oflow concentrations of chloride.

SUMMARY OF THE INVENTION

The present invention is a method of removing chloride fromphosphorus/vanadium/oxygen mixed oxide oxidation catalysts comprisingtreating a chloride containing catalyst comprising phosphorus, vanadium,other promoters and oxygen with stream of gas comprising from greaterthan 0 to less than 100% oxygen, from greater than 0 to less than 100%steam and from greater than 0 to less than 100% of an inert gas at atemperature and for a period of time to substantially reduce the amountof chloride in the catalyst. Preferably the oxygen comprises 0.1 to 15.0vol. % of the gas stream; the steam comprise 0.5 to 90.0 vol. % of thegas stream; and the inert gas comprises 0.1 to less than 100.0 vol. % ofthe gas stream. Preferably the temperature of the treatment is from 250°to 420° C. for 0.5 to 24 hours.

The term "inert gas" as used herein means nitrogen, helium, argon ormixtures thereof. The preferred valence of the vanadium in the treatedcatalyst is in the range of 3.5 to 4.5 and more preferably 3.7 to 4.2.The present process is useful for removing not only large percentages ofchloride, e.g. in excess of 50%, usually 70 to over 90%, but in removingsubstantially all of the chloride even when only small quantities areinitially present.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a triangular graphical representation showing a preferredranges of steam, oxygen and inert content of the gas used in the presentinvention.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The present invention comprises a method of removing chloride from aphosphorous/vanadium/oxygen catalyst by treating the catalyst with astream of gas comprising oxygen, steam and an inert gas at a flow rateandtemperature and for a period of time to substantially reduce theamount of chloride in the catalyst. The present process is particularlysuitable forremoving low concentrations of chloride from the catalysts,i.e., initial concentrations of chloride below 2 weight percent,preferably less than 1 weight percent with the residual chloride contentbeing less than 500 ppm,preferably less than 350 ppm.

Preferred catalysts are produced by the process comprising reducingvanadium in the +5 valence state in a substantially anhydrous organicmedium to a valence of less than +5 and digesting said reduced vanadiuminconcentrated phosphoric acid. The resultant catalyst complex ischaracterized as a mixed oxide, however, the structure of the complexhas not been determined but may be conveniently represented by a formulasuch as:

    VP.sub.a Me.sub.y O.sub.x

a is 0.90 to 1.3. Me is a metal, alkali metal or alkaline earth metal asknown in the art as modifiers for catalysts of this type. Thisrepresentation is not an empirical formula and has no significance otherthan representing the atom ratio of the components of the catalyst. Thex and y in fact, have no determinate value and can vary widely dependingon the combinations within the complex. That there is oxygen present isknown, and the O_(x) is representative of this. Suitable driedcatalystshave a crystallinity of 30 to 90%, preferably at least 40%.

In a preferred embodiment the improved catalyst comprise in addition toP, V and O, Zn, Li, and Mo is that produced from an alcoholic solutionreduction of vanadium pentoxide wherein the organic solvent is analcohol and the reduction of the vanadium is obtained by contacting itwith HCl. This is conveniently carried out by passing gaseous HClthrough the alcohol having the vanadium pentoxide suspended therein. Thevanadium pentoxide is reduced by the HCl and brought into solution asthe vanadyl chloride. The completion of the reduction is the appearanceof a dark reddish brown solution. Hydrogen bromide would be about thesame as a reducing agent in this system. It is preferred that thereduction temperature should be maintained at no greater than 60° C. andpreferably less than 55° C. Optimally active catalyst are the resultwhen the reduction is carried out temperatures in the range of about 35°C. to 55° C., preferably 37° C. to 50° C.

Generally in the catalyst preparation from 2500 to 4400 ml of alcohol,preferably 2700 to 4200 ml per pound of V₂ O₅ and from 1.5 to 3.0 poundsof HCl per pound of V₂ O₅ are employed.

To obtain the mixed oxides of vanadium and phosphorus, phosphoric acidof approximately 99% H₃ PO₄ (98 to 101%) is added, for example, preparedfrom 85% H₃ PO₄ and P₂ O₅ or commercial grades of 105 and 115%phosphoric acid diluted with 85% H₃ PO₄ or water to the final requiredconcentration of H₃ PO₄ and the vanadium compound digested which isdiscerned by a change in the color of the solution to a dark blue green.The alcohol is then stripped off to obtain the dried catalyst.

The digestion of the vanadium compound in the phosphoric acid isnormally conducted at reflux until the color change indicated thecompleted digestion.

The final removal of alcohol is usually carried out in an oven at atemperature in the range of 110° to 170° C. Reduced pressurecan also beapplied to lower oven temperatures. Generally calcination or roasting ofthe dried catalyst will be at a temperature in the range of 200° to 400°C. for a sufficient period to improve the catalytic properties of thecomposition.

The temperatures employed are relatively low hence the term calcinationmaynot be appropriate. In any event, heating the composition under thesetemperature conditions has been found beneficial. The calcination ispreferably carried out to produce materials having a characteristicpowderx-ray diffraction ratio.

The resultant preferred catalyst complex is characterized as a mixedoxide,however, the structure of the complex has not been determined butmay be conveniently represented by a formula such as:

    VP.sub.a Zn.sub.b Mo.sub.c Li.sub.d O.sub.x

a is 0.90 to 1.3, b is 0.001 to 0.15, c is 0.005 to 0.10 and d is 0.001to 0.15.

The organic solvent is preferably a primary or secondary alcohol such asmethanol, ethanol, 1-propanol, 2-propanol, butanol, 2-butanol,2,methyl-1-propanol, 3-methyl-2-butanol, 2,2-dimethyl-1-propanol,1-hexanol, 4-methyl-1-pentanol, 1-heptanol, 4-methyl-1-hexanol,4-methyl-1-heptanol, 1,2-ethanediol, glycerol, trimethylopropane,4-methyl2-pentanone, diethylene glycol and triethylene glycol ormixtures thereof. The alcohol is also a mild reducing agent for thevanadium +5 compound. A preferred cosolvent system comprises 2-butanoland from 5-50 vol % of the cosolvent.

Generally the atomic ratio of Zn to vanadium is in the range of 0.001 to0.15:1, however it has been found that lower ratios of zinc/vanadiumproduce the most active catalyst and compositions containing Zn/V moleratio in the range of 0.01 to 0.07 are preferred.

The phosphorus is generally present in these catalyst as well as thoseof the prior art in the mole ration of P/V 0.09-1.3/1. Optimum ratiosP/V arefound to be below 1.22/1 and above 1.0/1. The stabilizing effectof Mo allows the use of less phosphorus than otherwise comparable priorart catalyst and the concomitant benefit that phosphorus loss and theresulting deactivation of the catalyst in reactor operation is reduced,i.e., longer time trend (reactivity vs hours on stream).

The lithium component is present at an atomic ratio of 0.001 to 0.15:1,Li:V.

The lithium and zinc modifier components are added as the compoundsthereofsuch as acetates, carbonates, chlorides, bromides, oxides,hydroxides, phosphates and the like e.g., zinc chloride, zinc oxide,zinc oxalate, lithium acetate, lithium chloride, lithium bromide,lithium carbonate, lithium oxide, or lithium orthophosphate and thelike.

The molybdenum compound may dissolved in an organic solvent, asdescribed above or water and added to the reaction mixture. The solventcontaining the molybdenum compound may be added either with the othermodifiers or atdifferent time. If water is used the solvent containingthe molybdenum compound is preferably added after the first digestionand prior to the second digestion. The use of a soluble molybdenumcompound dissolved in a solvent according to the present invention foraddition to the reaction mixture has been found to be particularlyeffective in dispersing the molybdenum throughout the mixture and thefinal dried catalyst. Some examples of suitable soluble molybdenumcatalyst include phosphomolybdic acid, ammonium molybdate (VI)tetrahydrate, lithium molybdate, molybdenum tetrabromide, molybdenumtrioxyhexachloride and the like.

The catalyst may be employed as pellets, disc, flakes, wafers, or anyotherconvenient shape which will facilitate its use in the tubularreactors employed for this type of vapor phase reaction. For example thecatalyst may be prepared as tablets having a hole or bore therethroughas disclosedin U.S. Pat. No. 4,283,307 which is incorporated herein. Thematerial can be deposited on a carrier. Although fixed bed tubularreactors are standard for this type of reaction, fluidized beds arefrequently used foroxidation reactions, in which case the catalystparticle size would be on the order of about 10 to 150 microns.

The use of this class of catalyst for the partial oxidation of C₄ -C₁₀hydrocarbons to the corresponding anhydrides is generally recognized.They have been widely considered for the conversion of normal C₄hydrocarbons, both the alkane, n-butane, and alkene, n-butene, fortheproduction of maleic anhydride, which has a wide commercial usage.

The oxidation of the n-C₄ hydrocarbon to maleic anhydride may beaccomplished by contacting, e.g., n-butane in low concentrations inoxygenwith the described catalyst. Air is entirely satisfactory as asource of oxygen, but synthetic mixtures of oxygen and diluent gases,such as nitrogen, also may be employed. Air enriched with oxygen may beemployed.

The gaseous feed stream to the standard tubular oxidation reactorsnormallywill contain air and about 0.5 to about 2.5 mole percenthydrocarbons such as n-butane. About 1.0 to about 2.0 mole percent ofthe n-C₄ hydrocarbon are satisfactory for optimum yield of product forthe process of this invention. Although higher concentrations may beemployed, explosive hazards may be encountered except in fluidized bedreactors where concentrations of up to about 4 or 5 mole % can be usedwithout explosive hazard. Lower concentrations of C₄, less than aboutone percent, of course, will reduce the total productivity obtained atequivalent flow rates and thus are not normally economically employed.

The flow rate of the gaseous stream through the reactor may be variedwithin rather wide limits but a preferred range of operations is at therate of about 50 to 300 grams of C₄ per liter of catalyst per hour andmore preferably about 100 to about 250 grams of C₄ per liter of catalystper hour. Residence times of the gas stream will normally be lessthanabout 4 seconds, more preferably less than about one second, and down toa rate where less efficient operations are obtained. The flow ratesandresidence times are calculated at standard conditions of 760 mm. ofmercuryand at 25° C. A preferred feed for the catalyst of the presentinvention for conversion to maleic anhydride is a n-C₄ hydrocarboncomprising a predominant amount of n-butane and more preferably at least90 mole percent n-butane.

A variety of reactors will be found to be useful and multiple tube heatexchanger type reactors are quite satisfactory. The tubes of suchreactorsmay vary in diameter from about 1/4 inch to about 3 inches, andthe length may be varied from about 3 to about 10 or more feet. Theoxidation reaction is an exothermic reaction and, therefore, relativelyclose control of the reaction temperature should be maintained. It isdesirable to have the surface of the reactors at a relatively constanttemperature and some medium to conduct heat from the reactors isnecessary to aid temperature control. Such media may be Woods metal,molten sulfur, mercury, molten lead, and the like, but it has been foundthat eutectic salt baths are completely satisfactory. One such salt bathis a sodium nitrate-sodium nitrite-potassium nitrite eutectic constanttemperature mixture. An additional method of temperature control is touse a metal block reactor whereby the metal surrounding the tube acts asa temperatureregulating body. As will be recognized by one skilled inthe art, the heat exchange medium may be kept at the proper temperatureby heat exchangers and the like. The reactor or reaction tubes may beiron, stainless steel, carbon-steel, nickel, glass tubes such as Vycorand the like. Both carbon steel and nickel tubes have excellent longlife under the conditions for the reactions described herein. Normally,the reactors contain a preheat zone of an inert material such as 1/4inch Alundum pellets, inert ceramic balls, nickel balls or chips and thelike, present at about one-half to one-tenth the volume of the activecatalyst present.

The temperature of reaction may be varied within some limits, butnormally the reaction should be conducted at temperatures within arather critical range. The oxidation reaction is exothermic and oncereaction is underway,the main purpose of the salt bath or other media isto conduct heat away from the walls of the reactor and control thereaction. Better operations are normally obtained when the reactiontemperature employed is no greaterthan about 100° C. above the salt bathtemperature. The temperature in the reactor, of course, will also dependto some extent upon the size of the reactor and the C₄ concentration.Under usual operating conditions, in a preferred procedure, thetemperature in the center of thereactor, measured by thermocouple, isabout 365° C. to about 550° C. The range of temperature preferablyemployed in the reactor, measured as above, should be from about 380° C.to about 515° C. and the best results are ordinarily obtained attemperatures from about 380° C. to about 430° C. Described another way,in terms of salt bath reactors with carbon steel reactor tubes about 1.0inch in diameter, the salt bath temperature will usually be controlledbetween about 350° C. to about 550° C. Under normal conditions, thetemperature in the reactor ordinarily should not beallowed to go aboveabout 470° C. for extended lengths of time because of decreased yieldsand possible deactivation of the catalyst.

The reaction may be conducted at atmospheric, super-atmospheric or belowatmospheric pressure. The exit pressure will be at least slightly higherthan the ambient pressure to insure a positive flow from the reaction.Thepressure of the inert gases must be sufficiently high to overcome thepressure drop through the reactor.

The maleic anhydride may be recovered in a number of ways well known tothose skilled in the art. For example, the recovery may be by direct:condensation or by adsorption in suitable media, with subsequentseparation and purification of the maleic anhydride.

Examples 1-7 EXAMPLE 1

The following typical catalysts preparative procedure illustrate typicalcatalyst work up using the information discussed above. The tablets weremade as follows:

3600 ml anhydrous isobutanol and 636 grams V₂ O₅ were charged into a 2gallon Pfaudler reactor equipped with a mechanical stirrer, a gasinlettube, thermowell, Dean stark trap with a condenser, and a heatingjacket. About 3.5 lb hydrogen chloride gas were bubbled into the stirredsuspension. The reaction temperature was maintained at 40±3° C. To theresulting red-brown solution was added 9.5 grams anhydrous zincchloride, 2.96 grams lithium chloride, 13.10 grams molybdenum trioxideanda solution of 794.8 grams of 99.3% phosphoric acid. An additional1223 ml of anhydrous isobutanol were added to the reaction mixture, theratio of gal isobutanol/lb V₂ O₅ being about 0.91. The resultingsolutionwas refluxed for 1 hour. At the end of this digestion period thealcohol was stripped until about 3600 ml distillate were removedresulting in a thick slurry. The thick slurry was then dried in an ovenfor 16 hours at 150° C. The dry cake was then crushed and calcined at260° C. About 4% graphite was added and the graphite containing powderwas usedto fabricate 3/16"×3/16" tablets with a 1/16" hole strucktherethrough. These tablets were used as the virgin V/P/O promotedcatalysts in Examples 8 and 9.

EXAMPLE 2

The following method was used to synthesize the powdered catalysts whichwere treated for chloride removal in Examples 3-7.

The catalyst was prepared as in Example 1 except that the ratio of galisobutanol/lb V₂ O₅ was about 0.829. The thick slurry obtained afterdistilling off the isobutanol was dried and calcined as in Example 1.The calcined powder containing about 4% graphite was used in Examples3-7.

EXAMPLES 3-7

These examples illustrate the removal of chloride from catalyst in apowderform prepared in Example 2.

About each 15 g of powder catalyst prepared in Example 2 were placed ina 22 mm ID quartz tube reactor which was heated by a Lindberg furnace. Aflow of gas containing the desired components was heated and passedthrough the reactor. The original level of chloride in the sample was0.7 wt %. The conditions and the amount of chloride removal are reportedin TABLE I.

                  TABLE I                                                         ______________________________________                                        EXAM.        3       4       5     6     7                                    ______________________________________                                        TEMP. °C.                                                                           375     375     375   375   375                                  TIME, HRS.   3       3       3     3     3                                    % O.sub.2.sup.1                                                                            8       4       1.9   0     21                                   % STEAM.sup.2                                                                              46      43      33    30    0                                    V.sup.ox     4.2     4.0     4.0   3.5   4.5                                  % Cl REMOVED 97.9    91.1    75.4  50.6  70.0                                 ______________________________________                                         .sup.1 % OXYGEN IN THE GAS STREAM BEFORE STEAM WAS INTRODUCED, THE BALANC    IS NITROGEN.                                                                   .sup.2 % STEAM OF THE TOTAL GAS FLOW THROUGH THE CATALYST BED.           

EXAMPLES 8 AND 9

These examples illustrate the removal of chloride from a catalyst in atablet form as prepared in Example 1.

The procedures of Example 3-7 were followed except that instead of thepowders, the treatments were made on full tablets prepared in Example 1.The original level of chloride in the sample was 0.7 wt %. Theconditions and the amount chloride removed and other results arereported in TABLE II.

                  TABLE II                                                        ______________________________________                                        EXAM.              8       9                                                  ______________________________________                                        TEMP. °C.   375     375                                                TIME, HRS.         3       3                                                  % O.sub.2.sup.1    3.5     3.5                                                % STEAM.sup.2      32      40                                                 V.sup.ox           4.0     4.1                                                % Cl REMOVED       90.3    95.4                                               ______________________________________                                         .sup.1 % OXYGEN IN THE GAS STREAM BEFORE STEAM WAS INTRODUCED, THE BALANC    IS NITROGEN.                                                                   .sup.2 % STEAM OF THE TOTAL GAS FLOW THROUGH THE CATALYST BED.           

The FIGURE is a graphic representation of the most preferable ranges forsteam, oxygen and nitrogen (inert). A suitable range according topresent invention for combinations of steam, oxygen and inert fallwithin the areaA-B-C on the triangular graph. The valence of thevanadium can range from 3.5 to 4.5, preferably 3.7 to 4.2. A preferredrange falls within the areaA-B'-C in FIG.1. The points A, B' and C allrepresent combinations of steam, oxygen and inert content that resultedin excellent chlorine removal (at least 75%) and yet left the vanadiumin an oxidation state of from 3.9 to 4.2.

The virgin catalyst may be calcined as a powder, for example in an ovenor in a fluidized bed; as a shaped article, for example as extrudates orpills; in a reactor or ex situ, prior to contact with hydrocarbon feed.

The reduced chloride content of the catalyst made according to thepresent invention is a significant improvement since chlorine in thecatalyst tends to deteriorate reaction vessels and downstream equipment.

The invention claimed is:
 1. A method of removing chloride fromphosphorus/vanadium/oxygen mixed oxide oxidation catalysts comprisingtreating a chloride containing virgin catalyst wherein said chloride isintroduced into said catalyst by using HCl as a reducing agent duringthe preparation of said catalyst and comprising phosphorus, vanadium,oxygen and up to 2.0 wt % chloride with a stream of gas comprising 0.1to 15 vol % oxygen, 0.5 to 80 vol % steam and 0.1 to 80 vol % of aninert gas selected from the group consisting of nitrogen, helium, argonand mixtures thereof at flow rate and temperature and for a period oftime to remove over 50% of chloride in the catalyst.
 2. The methodaccording to claim 1 wherein the temperature of the treatment is in therange of from 250° to 420° C.
 3. The method according to claim 1 whereinthe treatment is in the range of 0.5 to 24 hours.
 4. The methodaccording to claim 1 wherein the valence of the vanadium is between +3.5and +4.5.
 5. The method according to claim 1 wherein the valence of thevanadium is between +3.7 and +4.2.
 6. The method according to claim iwherein said inert gas comprises nitrogen.
 7. The method according toclaim 1 wherein the phosphorus/vanadium/oxygen mixed oxide oxidationcatalysts comprise a composition of the formula:

    VP.sub.a Zn.sub.b Mo.sub.c Li.sub.d O.sub.x

wherein a is 0.90 to 1.3, b is 0.001 to 0.15, c is 0.005 to 0.10 and dis 0.001 to 0.15 and x is an determinate number.
 8. The method accordingto claim 1 wherein the catalyst is treated in the form of a powderbefore being fabricated into final tableted form.
 9. The methodaccording to claim 1 wherein the catalyst is treated in the form oftablets or other shapes.
 10. A method of removing chloride fromphosphorus/vanadium/oxygen mixed oxide oxidation catalysts comprisingtreating a chloride containing virgin catalyst prepared using HCl as thereducing agent for the vanadium and comprising phosphorus, vanadium,oxygen and up to 1.0 wt % chloride with a stream of gas comprising 0.1to 15 vol. % oxygen, 0.5 to 90 vol. % steam and 0.1 to less than 100vol. % nitrogen at a temperature in the range of from 250° to 420° C.for 0.5 to 24 hours and to reduce the amount of chloride in the catalystfrom 70 to over 90%.
 11. The method according to claim 10 wherein thephosphorus/vanadium/oxygen mixed oxide oxidation catalysts comprise acomposition of the formula:

    VP.sub.a Zn.sub.b Mo.sub.c Li.sub.d O.sub.x

wherein a is 0.90 to 1.3, b is 0.001 to 0.15, c is 0.005 to 0.10 and dis 0.001 to 0.15 and x is an determinate number.